The invention relates to an over-center hinge for cabinet doors, having a door-related hinge part in the form of a sunken cup in which one end of each of two links is pivoted while the other ends are pivoted on a jamb-related hinge part in the form of an elongated supporting arm so as to form a rectangular articulation. On the side of the inner link facing the outer link, and between its pivot eyes, a cam is pivotally mounted with its free end pointing toward the supporting arm. The contact surface of this cam is urged by a spring inserted between the cam and the pivot eye provided on the supporting arm end of the inner link against a counter-surface formed by a section of the inner wall of the supporting arm such that in its closing movement, after a dead center is passed, the hinge is resiliently forced to the closed position.
Over-center hinges of this kind are known (DE-PS No. 25 16 084) and have proven practical on account of the compact and thus space-saving construction and the reliable operation of its over-center mechanism. The spring-biased cam is, in these known hinges, made from a plastic that has been optimized in regard to its frictional and wear-resistant properties and which has excellent stability as long as the metal counter-surface formed on the supporting arm is not very rough, i.e., if it is sufficiently smooth. In the case of the supporting arm stamped from sheet metal which is widely used, this requirement of sufficient smoothness of the counter-surface is satisfied easily, as a rule, if it is used in the bright form which it has when removed from the press die. To achieve a certain appearance, however, sheet metal supporting arms are often handled afterward such that their surface no longer has the good surface quality that is desired for cooperation with the face of the cam. An example is the sandblasting of the supporting arms, or the forming of galvanically dulled surfaces to achieve a uniformly matte appearance. These procedures roughen the supporting arm surface, including the area of the counter-surface, and a rough surface texture of this kind is less suited for cooperation with the contact surface of the plastic cam, because it acts like a very fine file, i.e., it removes material from the contact surface and thus gradually changes the movement and hence also the operating characteristic of the hinge. Even in the case of lacquered supporting arms the lacquer is applied to intentionally roughened surfaces to promote good adhesion. In the area applied to the counter-surface the lacquer, however, wears through in a relatively short time, so that then a rough metal surface is again cooperating with the contact surface of the plastic cam. To avoid the above-described disadvantages it would indeed be possible to smooth the supporting arm in the area of the counter-surface by polishing, but this additional step in the manufacture of the hinge would unacceptably increase the cost, since an automatic sorting and polishing apparatus would have to be developed for the partial finish polishing of the counter-surface, which would call for an unreasonable investment. Furthermore, the problem would also exist that the metal particles removed by polishing would adhere to the adjoining areas of the rough metal surface, so that another cleaning operation would be necessary.
The invention therefore sets for itself the task of improving the known hinge such that its over-center mechanism will operate reliably in the desired manner throughout its useful life when the supporting arm has been manufactured with an undesirably rough surface at the wall area that cooperates with the cam.